As a powerful analysis technology, mass spectrometry can realize qualitative and quantitative analysis of compounds, which is applied to such fields as pharmaceutical analysis, environment monitoring, national security, medical jurisprudence and proteomics. It is well known that tandem mass spectrometry (Tandem MS) is available for characterization and analysis of compound structure. Specific tandem mass spectrometric analysis process is stated as follows: The first stage aims at isolation, at which ions of certain mass-to-charge ratio (m/z) are selected from samples to be analyzed for isolation; isolated ions will become parent ions; the second stage aims at collision induced disassociation (CID); parent ions are to be in collision with neutral molecules of such gases as helium, argon and nitrogen; energy produced by collision is to be deposited on parent ions, and thereby enhance intrinsic energy of parent ions; eventually, parent ions will subject to fragmentation to obtain fragment ions; at the third stage, mass spectrometry peak of fragment ions is to be obtained through mass analysis to complete MS/MS analysis. In the event that ions of certain mass-to-charge ration are to be selected from fragment ions for isolation, they will be taken as parent ions to repeat aforesaid process until multi-stage mass spectrometric analysis is achieved. CID is the most extensive and comprehensive disassociation technology.
Among various spectrometers, quadrupole spectrometer and quadrupole ion trap spectrometer are recognized as the most appropriate devices for collision induced disassociation. Among them, quadrupole spectrometer is also known as quadrupole mass filer, which is only available for passing of ions of certain mass; therefore, numerous quadrupoles are to be spatially connected in series in case of tandem mass spectrometric analysis within the quadrupoles; normally, combination of three-stage quadrupoles, namely triple quadrupoles, is used. Triple quadrupole mass spectrometer is normally provided with larger area. Quadrupole ion trap (QIT) can execute such procedures as isolation, disassociation and mass analysis of ions in one trap, which enjoys unique advantages over tandem spectrometry.
According to its working principle, ion trap mass analyzer is expected to obtain movement status and results of ion of certain mass-to-charge ratio in the electric field based on solution to Mathieu quadratic linear differential equation set. Mathieu equation is obtained based on the fact that action of electric field on charged ions in ion trap is in compliance with Newton's Second Law, which aims to describe movement track and results of ions in quadrupole electric field. Taking 3D ion trap for instance, the following formula is obtained based on solution to Mathieu Equation:
      a    =                  16        ⁢        eU                              m          ⁡                      (                                          r                0                2                            +                              2                ⁢                                  z                  0                  2                                                      )                          ⁢                  Ω          2                      ,      q    =                  8        ⁢                                  ⁢        eV                              m          ⁡                      (                                          r                0                2                            +                              2                ⁢                                  z                  0                  2                                                      )                          ⁢                  Ω          2                    
In the formula, a refers to a trap parameter in direct proportion to DC voltage; q refers to a trap parameter in direct proportion to radio voltage; U refers to DC voltage imposed on ion trap pole; V refers to radio frequency voltage imposed on ion trap electrode; Ω refers to frequency of radio frequency voltage; r0 refers to radius of ring electrode; z0 refers to axial radius. Ions of different mass-to-charge ratio escaped from ion trap are to be detected once alteration to electric field is made on ion trap electrode. Ions moving inside the ion trap are stable or within the stable area. Ions escaped from the ion trap are instable or outside of stable area. According to analysis based on stability diagram, ions of different mass-to-charge ratios will move out of the stable area in proper sequence under the action of electric field with sequential variations in case of mass analysis of ion trap; in other words, they are ejected from the ion trap and detected by ion detector outside of the trap to complete mass analysis.
Resonance excitation technology has become an ion ejection and disassociation approach widely applied to the ion trap after sustainable development for nearly 20 years. Normally, resonance excitation is realized by using a pair of electrodes in the ion ejection direction inside the ion trap to impose an auxiliary AC voltage, namely dipolar excitation voltage; such voltage is provided with specific frequency and amplitude; whereas voltage amplitude and frequency on the pair of electrodes are identical with phase difference up to 180°. Ion confined inside the ion trap are provided with a secular frequency (ω) under the action of radio frequency voltage; ions of different mass-to-charge ratio are provided with different secular frequencies. Interrelation between secular frequency and frequency (Ω) of radio frequency voltage is stated as follows:
  ω  =            β      2        ⁢    Ω  
β is a coefficient as well as a parameter as shown in stability diagram for ion trap; the two are mutually associated. When frequency of dipolar excitation voltage is identical to secular frequency of ion of certain mass-to-charge ratio, the ion is to subject to resonance to intensify its movement in the direction of dipolar excitation voltage; eventually, ion ejected from small hole or slit on the electrode is to be collected by the ion detector. When frequency of dipolar excitation voltage is deviated from secular frequency of ion of certain mass-to-charge ratio, resonance is still available despite of significantly reduced amplitude that is inadequate to eject the ion; under such circumstance, resonance of ion at low amplitude may result in intensified collision between ion and neutral gas molecules in the trap to complete collision induced disassociation. Frequency, amplitude and duration of dipolar excitation voltage may without exception affect results of collision induced disassociation. Resonance excitation technology still has its disadvantages and deficiencies despite of its relatively high fragmentation efficiency. The underlying reason is that only ions of certain fixed mass-to-charge ratio are available for resonance, and mass-to-charge ratio of fragment ions obtained through fragmentation is to be changed, namely increased or decreased; at this point, secular frequency of fragmentation ions is different from AC frequency, and is unavailable for resonance; in other words, as it is unavailable for further disassociation, fragment information as shown in the tandem mass spectrogram is to be restricted.
Non-patent literature 1 and 2 introduce a method used to realize tandem mass spectrometry; in other words, dipolar DC voltage is to be imposed on a pair of electrodes. When ion of certain mass-to-charge ratio is isolated, dipolar DC voltage is to be imposed; under the action of DC voltage, the ion is to be deviated from the trap center to accelerate its movement; meanwhile, radio frequency voltage still has certain heating effect on this ion. Eventually, it may result in significant increase in intrinsic energy of the ion and disassociation. As collision induced disassociation realized by dipolar DC voltage is not in resonance mode, which has no restrictions on mass-to-charge ratio of ion, ion may subject to further disassociation under the action of dipolar DC even if parent ion becomes fragmented; as a result of it, information on fragmentation peak as shown in tandem spectrogram will be more abundant; Different from conventional resonance excitation approach, collision induced disassociation driven by dipolar DC voltage is a non-resonance excitation approach that can obtain more abundant information on fragmentation ions; it is an important innovation on existing disassociation approach. However, such approach requires an additional DC power to supply DC voltage so as to provide dipolar DC voltage via the electric circuit, meanwhile, as dipolar DC voltage subject to sequential variation, and required precise control, it has more stringent and complicated requirements for hardware of instruments.    Non-patent literature 1: B. M. Prentice, W. Xu, Z. Ouyang, S. A. McLuckey, DC potentials applied to an end-cap electrode of a 3D ion trap for enhanced MSn functionality. International Journal of Mass Spectrometry 2011, 306, 114-122.    Non-patent literature 2: B. M. Prentice, S. A. McLuckey, Dipolar DC Collisional Activation in a “Stretched” 3-D Ion Trap: The Effect of Higher Order Fields on rf-Heating. Journal of the American Society for Mass Spectrometry 2012, 23, 736-744.